1. Field of the Invention
The present invention relates in general to a data transmitting and receiving system, and in more particular to a communicatory navigation system comprising an information center and a plurality of on-vehicle navigation units each communicatively connected to the information center.
2. Description of the Prior Art
The most important function of the navigation system mounted on a vehicle is of course to navigate the vehicle to drive from the current position to a predetermined destination along a suitable drive route. Most of conventional navigation systems have a control unit installed in the vehicle and data or information to be used for such drive route navigation are stored in a memory connected to the control unit, which may not be sufficient to store the increasing data that is necessary or at least desirable for smooth navigation in various situations.
To cope with this problem, a communicatory navigation system has recently been proposed and put to practical use. In the communicatory navigation system, there is installed an information center with a database memory of a great capacity from which supplementary data is transmitted to the on-vehicle navigation unit to thereby assist the drive route navigation. More particularly, the navigation unit obtains the current vehicle position data detected by a position sensor and the destination position data inputted by the user with any suitable input device, and transmits these items of data to the information center. Based on these items of position data received and in reference to the road data in the database memory, the information center makes a plan for a recommended drive route from the current position to the destination, and transmits the planned drive route back to the navigation unit. The navigation unit receiving the drive route from the information center will operate to display the drive route on a display and induce the driver to drive therealong.
There is an increasing demand of the navigation system that a drive route should be determined not only depending upon the current and destination position data but also by taking into consideration various conditions in or around the vehicle (referred to as "vehicle conditions" throughout the specification) such as weather conditions, road surface conditions, traffic jams, roads under construction, etc. For example, if there is a heavy traffic jam in the course of a specific drive route that is determined by normal discrimination, it is preferable to arrange another route bypassing the traffic jam point so that the driver is guided therealong to drive smoothly and reach the destination earlier. To achieve this, it is necessary that data indicating the vehicle condition, for example data indicating ON/OFF of a wiper and the past drive route data of the vehicle, is transmitted from the navigation unit to the information center. However, while the current and destination position data is being transmitted from the navigation unit to the information center, or while the drive route is transmitted from the information center to the navigation unit, so that the communication line or telephone line therebetween is busy, the same line can not be used for other data transmission. Another communication medium such as a hand phone may be used for transmission of the drive condition data, but in practice this is not preferable because it takes another communication charge and requires the user's calling operation.
Another problem in the prior art communicatory navigation system is a matter of version-up or updating of the road database. Although the information center may store a lot of the road database, a newly constructed road is not in the database unless the database is updated to include such a new road. Until the database is updated, no drive route including the new road can be determined by the information center.